A novel thermal switch and variable capacitance implement by CMOS MEMS process approaching in micro electrostatic converter

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A novel thermal switch and variable capacitance implement by CMOS MEMS

process approaching in micro electrostatic converter

Jin-Chern Chiou

a

_{, Lei-Chun Chou}

a,⇑

_{, You-Liang Lai}

b

_{, Sheng-Chieh Huang}

a a

Dept. of Electrical Control Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC b

National Chip Implementation Center (CIC), Hsinchu 300, Taiwan, ROC

a r t i c l e

i n f o

Article history:

Available online 13 June 2012 Keywords: CMOS MEMS Thermal switch Variable capacitance PDMS

a b s t r a c t

This paper focuses on implementing a novel thermal switch and variable capacitance design by using commercially available CMOS MEMS process which can approach in a micro electrostatic converter sys-tem. In this system, there are two major parts. First is the variable capacitance, and the second is the ther-mal switch. In the variable capacitance, it implement by UMC 0.18lm one-poly seven-metal (1P7M) CMOS MEMS process. In the post process, the silicon-oxidation has been released and the gap between two metal layers ﬁlled with PDMS (Polydimethylsiloxane). Filling with PDMS is to signiﬁcantly increase Cmax.

In the thermal switch design, there are two novel designs in this switch: first, the soft contact structure and post-processing fabrication; second, using residual stress to achieve large structural deformation and a new design of thermal switch. To create the soft contact structure, residual stress effect has been uti-lized to make different bending curvatures. According to the experiments, the layer metal 1 has the larg-est residual stress[1]effect that can achieve the largest deflection in z-axis. Because the residual stress of the layer metal 1 is negative, the structure will bend down after release, the largest contact area which has been set up to get the lowest contact miss ability. In the post-processing fabrication, 0.3lm thickness gold will be patterned on the contact tips. Due to gold, rather than Aluminum, has no oxidation issue, it has more reliability on preventing the problem of oxidation than Aluminum. In the new thermal actuator design, we design a novel folded-flexure[2,3]with the electro-thermal excitation to turn the switch on or off. In the prototype, the device size is 500lm 400lm and the gap between two contact pads is 9lm in off-state.

Depending on the experimental results, the Cmaxis 19.22 nF, and the Cminis 10.65 nF in variable

capac-itance. The switch can work stably at 3 V, and the working temperature and operating bandwidth are individually 20–200 °C. The natural frequency of the switch is 42.9 kHz and the response time is 14.28 Hz.

1. Introduction & system

Micro-Electro-Mechanical System (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a com-mon substrate through micromachining processes compatible with conventional integrated circuit (IC) fabrication. By combining sili-con based micro-machines with microelectronics, MEMS technol-ogy has revolutionary impact on all categories of applications, making possible the realization of the complete system on chip (SOC) concept. Current MEMS research and development has al-ready been applied in ﬁelds such as micro-optical systems, sensors and actuators, micro ﬂuidic elements, and even biomedical appli-cations, all with fruitful results. Such continuous improvement of

microsystem technology promotes the development of smart micro transducer networks, such as RFID (Radio Frequency Identi-ﬁcation) and wireless sensor network. These highly integrated por-table devices have received increasing interest in recent years. Nevertheless, power consumption has become a severe limitation on the development due to the limited energy capacity of the small volume energy storage devices. Traditional storage devices include batteries, micro batteries, micro fuel cells, ultra capacitors, micro heat engines, and radioactive materials. Researchers attempt to increase the energy density in these storage devices, but the solutions still have ﬁnite lifetime and high maintenance costs. Fortunately, the advance in low power VLSI (Very Large Scale Inte-grated circuit) technology, along with the low duty cycles of the wireless sensor networks, have reduced power requirements to tens to hundreds of micro watts. It becomes possible to power these portable devices by scavenging ambient energy from the environment, thus providing a self renewable or even self

http://dx.doi.org/10.1016/j.sse.2012.05.014 ⇑Corresponding author.

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sustainable energy source which can replenish part or all of the consumed power. This concept has received attention along with the development of wireless sensor networks, and research on var-ious ambient energy scavenging technologies are being conducted. Electrostatic capacitive energy conversion utilizes a variable capacitor to convert vibration energy into electric energy. The elec-trical energy W stored in a capacitor with capacitance C and volt-age V is W ¼1 2CV 2 ¼1 2 Q2

C. If the capacitance of a pre-charged

capacitor with constant charge Q is decreased due to vibration, the stored electrical energy in the capacitor will increase, thus con-verting the kinetic energy into electrical energy. The main concern of the capacitive energy conversion is how to extract the stored electrical energy in a properly controlled timing scheme. Capaci-tive energy conversion is more suitable for the steady frequency moderate amplitude vibration source mentioned before.

Fig. 1is the micro electrostatic energy converter system. The converter is composed of an auxiliary battery supply Vin, a

vibra-tion driven variable capacitor Cvand an output storage capacitor

Cstor, which is connected to the load RL. Two switches, SW1 and

SW2, are used to connect these components and control the

charge–discharge conversion timing. The variable capacitor serves as the conversion transducer and the auxiliary battery supply is used to pre-charge the capacitor. A basic operation cycle begins

Fig. 1. Micro electrostatic energy converter system.

Fig. 2. CMOS MEMS process.

Fig. 3. (a) Remove SiO2by HF vapor. (b) The gap ﬁlled with PDMS.

Table 1

Eight combinations of CMOS metal/oxide stacked structures.

No. Metal 1 Metal 2 Metal 3 Metal 4

1 d d d d 2 d d d 3 d d d 4 d d d 5 d d 6 d d 7 d d 8 d

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when the variable capacitor Cvis charged by the auxiliary voltage

supply Vinthrough SW1 at its maximum Cmax. After Cvis charged

to Vin, SW1 is opened and the capacitance changes from Cmaxto

Cmin due to vibration driven displacement. In this process, the

charge Q on the capacitor remains constant (SW1 and SW2 both open). Therefore, the terminal voltage on the capacitor is increased, converting the kinetic energy of vibration into electrical energy stored in Cv. When the capacitance reaches Cminand terminal

volt-age reaches Vmax, SW2 closes and allows Cstorto be charged by Cv

through charge redistribution, transfer the energy to the output port. SW2 is then opened and Cvvaries back to Cmax, preparing

for the next conversion cycle. 2. Fabrication

CMOS MEMS utilizes the CMOS stacked layers to form the mi-cro-sensors and the micro-actuators. It has great potential for

com-combination structures. It not only costs less testing area and sim-ulation time but also includes the stress of interlays. This paper used the effective mechanical properties extracted from eight com-binations of CMOS metal/oxide stacked structures to simulate and predict the static and dynamic behavior of MEMS device. The prob-lem of the lateral contact switch has been solved in this study by depositing gold on the Aluminum layer.

The CMOS MEMS multi-project wafer (MPW) was fabricated by TSMC 0.35

l

m two-poly four-metal (2P4M) CMOS process and CIC micromachining post process. The major materials of metallization and dielectric layer in this CMOS process are respectively alumi-num and silicon oxide. Fig. 2shows the cross-section views of the MPW with post-CMOS micromachining steps. By the way, in this study we use Bio CMOS MEMS process, a gold layer is depos-ited on top layer.

For alleviating the dry etching process on dielectric layers, as

Fig. 2a shows, the CMOS passivation layers on etching selective re-gions are removed during the CMOS process. Before the anisotropic dry etching, an additional photo-resist layer is spun and litho-graphed on the wafer for the post process. This lithography enables the deﬁnition of the protective region of photo-resist layer over the metal of the inductor. The thickness of the photo-resist requests at least 5

l

m to avoid the RIE etching directly on the top meal. The anisotropic RIE etching with CF4, CHF3 and O2 is subsequently used to remove the exposed dielectric oxide inFig. 2b. In this step, most photo-resist layers would exhaust with the RIE gas. When all metal layers are done, a gold layer is deposited on the top layer, as shown inFig. 5. AsFig. 2d indicates, the following isotropic dry etching with SF6 and O2 is then included to remove the underlying silicon substrate and release the microstructures.

3. Device design

3.1. Variable capacitance design

In the variable capacitance design, a out-plane gap-closing comb structure is used for the variable capacitor. It is implemented by UMC 0.18

l

m 1P7M CMOS process. In the post process, the sil-icon dioxide layer removed by HF vapor, thus there are seven metal layers can be used in variable capacitor. It means that there are up

6 12.45 6.00

7 5.42 3.90

8 10.39 4.85

Fig. 4. Effective gradient stress vs. metal layer.

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to 6 available variable capacitors in one block. In order to increase the capacitance value, the gap of the metal layer is filled with PDMS (polydimethylsiloxane), as shown inFig 3. The relative per-mittivity of PDMS is 10. For fill the PDMS into the gap of the vari-able capacitance, about 0.05 ml PDSM had drop at the etch-hole. Stand for 1 h the PDSM will fill the gap of the capacitance. 3.2. Mechanical switch design

The switches SW1 and SW2 are realized as lateral contact mechanical switches. Conventional design of the charge–discharge timing control switches utilize diodes or clocked active switches. In order to prevent charge leakage out of Cvand Cstor, the switches

must have a reverse leakage current lower than a few nA. This is

not common in commercially available diodes and other switching circuitry. Capacitive coupling is another problem, in which the capacitance of the switch contributes to parasitic capacitance. Our design of SW1 and SW2 has barely zero charge leakage and very low capacitance coupling effect. Other advantages are the low energy consumption, the synchronous operation to the vari-able capacitor, and the monolithic integration with the whole de-vice structure.

SW1 is designed to contact simultaneously when or merely after the metal layer has touched. SW2 is designed by a thermal type. SW2 is turn on at initial time, and there are current through pass the switch. The temperature of the switch will increase caused by the current. And it will pull-back, thus the SW2 will turn off. To design the thermal switch, there are two parts should discuss. First is the soft contact structure, and second is new design of thermal actuator.

3.3. The soft contact structure

In a normal switch design, the contact area is too small to make a poor contact. To solve this problem, a new design of the contact proﬁle which used negative residual stress effect and the different conﬁgurations of the contact tip has proposed in this study. Every metal used in the TSMC 0.35

l

m 2P4M CMOS process has its own effective residual stress. Combine different metal layer also can get different residual stress[1].Table 1shows eight combinations of CMOS metal/oxide stacked structures.

By comparingTable 2, only the layer Metal 1 has the negative and residual stress (Table 2andFig. 4). Thus the contact proﬁle will bend down after release. With this proﬁle, it provides lager contact area than the normal design.

This study also designs different conﬁgurations of the tip with different contact proﬁle which can get more contact area (Fig. 5a and b).

In the post-processing, 0.3

l

m thickness gold will be deposited on the surface of the element. Since gold is non-active, it has more reliability on avoiding the problem of oxidation and on increasing the conductivity at contact area.

3.4. A novel design of thermal switch

In the novel thermal switch design, this paper proposed a novel folded-ﬂexure with the electro-thermal excitation to control the

Fig. 6. (a) is the top view of micro-cantilever beam before release. (b) is the structure after release at 293 K (20 °C). (c) Due to different thermal coefﬁcient, the structure will pull back with temperature increase to 473 K (200 °C).

Table 3

Average values and standard deviations (STD) of effective gradient stress property.

No. Stress gradient (MPa/lm) STD

1 64.27 4.43 2 27.79 4.74 3 40.45 7.33 4 38.88 4.79 5 13.23 1.26 6 7.97 2.67 7 20.35 1.45 8 -20.96 3.07

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switch on or off, as shown inFig. 6. In the normal design[2,3], the numbers of the layers used are metal 1, metal 2 and metal 3. In this proposed design, the different numbers of layers are used.

In this design, effective gradient stress and thermal coefﬁcient are used ﬁnite-element-method software in simulation. By com-paringTable 3, the stress increases with the numbers of metal lay-ers, as shown in Fig. 7. It indicated that oxide layer in CMOS process has compressive stress and the metal layer has tensile stress before post-processing release, as illustrated inFig. 6a. Due to the isotropic dry silicon etch and structural release. The oxide layer has tensile stress and the metal layer has compressive stress after release, as shown inFig. 6b.

When the switch turn on, there are current pass by the switch and let the temperature of the switch rise up. Because oxide layer (8.5

l

m/K) and metal layer (27.4

l

m/K)[4]have different thermal coefficient. The folded-flexure of the switch will pull back and the switch will turn off, as shown inFig. 6c. So we can use this folded-flexure structure to control the switch turn on or turn off.

4. Simulation and experimental result

The simulation software (Coventor-Ware) also used to verify the switch and capacitance design. By the simulation result, the initial gap is 11

l

m at 293 K (20 °C). Due to temperature increase, the gap pulling back and the displacement of the contact-tip is 6

l

m at 473 K (200 °C), as shown inFig. 8.

Fig. 9is the layout of the variable capacitance, and it has imple-mented by UMC 0.18

l

m CMOS MEMS process, as shown in

Fig. 10a and b the SiO2layers of the capacitance have been etched

by HF and ﬁlled with the PDMS. The size of the capacitor is 1100

l

m 1100

l

m.Fig. 11is the thermal switch implement by TSMC 0.35

l

m 2P4M CMOS process and CIC micromachining post process. The width of the device is 500

l

m, and length is 400

l

m. The switch and variable capacitor were ﬁrst tested by a probe station to see if they were properly released. The switch was mea-sured by the MEMS Motion Analyzer (MMA G2, Umech Technolo-gies, USA) system at the National Chip Implementation Center (CIC). The MMA system utilizes image processing technique and measures the periodic relative motion between the movable and the still structures of the device. The photograph of the MMA mea-surement setup provided by CIC is shown inFig. 12. The MMA measurement results are plotted in Fig. 13a. The resonant fre-quency is about 42.9 kHz with a corresponding 90_{phase shift}

Fig. 8. (a) Thermal switch at 20 °C in Coventor-Ware. (b) Thermal switch at 200 °C in Coventor-Ware.

Fig. 9. Layout of the variable capacitor.

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at resonance. From the measured amplitude response curve, it is seen that the 3 dB bandwidth is aboutDf3dB= 250 Hz inFig. 13b.

The response time of the switch is 14.28 Hz, as shown inFig. 14. The system is used into bio-medical environment. The operation frequency is about 1–2 Hz, so the response time of the switch is suitable for the system.

In the static state of the switch, we use probes to control the voltage given and signal analysis.Fig. 15a is the photo by Optical Microscope (OM) andFig. 15b is the measurement result by optical proﬁler. The gap is 19.86

l

m at initial time (0 V and 300 K).Fig. 16a is the switch in 20 °C, andFig. 16b is the switch in 200 °C by using MMA.

5. Conclusion

In this study, a MEMS-based variable capacitor and a new ther-mal switch was proposed and implemented by using CMOS MEMS process and CIC micromachining post process has been developed. In the prototype, the device size of the variable capacitor is 1100

l

m 1100

l

m, and the thermal switch size is 500

l

m 400

l

m. The gap between two contact pads of the switch is 9

l

m in off-state. According to the measurement result and the photo by OM, the switch can work stably at 3 V, and the working temperature and operating bandwidth are individually 20–200 °C. The natural frequency of the switch is 42.9 kHz and the response time is 14.28 Hz.

Fig. 11. Device implemented by TSMC 0.35lm process.

Fig. 12. MEMS Motion Analyzer (MMA).

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Fig. 14. Response time of the switch.

Fig. 15. (a) One side of the switch. (b) Measurement result by optical proﬁler.

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Acknowledgments

This work was supported in part by National Science Council, Taiwan, under Contract 032, 100-2220-E-009-019, and in part by Taiwan Department of Health Clinical Trial and Research Center of Excellence under Contract Nos. DOH99-TD-B-111-004 and DOH99-TD-C-111-005. This work was also sup-ported in part by the UST-UCSD International Center of Excellence in Advanced Bio-engineering sponsored by the Taiwan National Science Council I-RiCE Program under Grant No. NSC-100-2911-I-009-101. This paper is particularly supported by ‘‘Aim for the Top University Plan’’ of the National Chiao Tung University and Ministry of Education, Taiwan, ROC. The authors would also like to thank National Chip Implementation Center (CIC) for chip fabrication.

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